The rhizosphere of plants is usually occupied by both deleterious rhiz
obacteria (DRB), which may inhibit plant growth, and plant growth prom
oting rhizobacteria (PGPR). Root exudates play a key role in determini
ng host specific interactions with, and the composition of, the rhizob
acterial population. A degree of coevolution between rhizobacteria and
plants is implied. DRB have been implicated in yield declines associa
ted with continuous monoculture. DRB inhibit growth primarily through
production of phytotoxins but phytohormone production, competition for
nutrients and inhibition of mycorrhizal function may also occur. The
effect of individual isolates of rhizobacteria on plants can fluctuate
from growth inhibition to growth promotion, according to environmenta
l conditions, the host genotype and mycorrhizal status. Furthermore, r
hizobacteria have the potential to act as both DRB and PGPR through si
multaneous inhibition of plant growth and control of fungal plant path
ogens. Therefore classification of rhizobacteria as either DRB or PGPR
is somewhat equivocal and should be based upon their net effects on p
lant growth according to circumstance. The capacity for fluctuation in
the phytogenic function of rhizobacteria presents a challenge for man
agement of DRB and manipulation of the rhizosphere to improve the sust
ainability of agriculture.